Obesity affects 39.8 percent of adults in the United States, according to government data; in Australia, two-thirds of adults (67 percent) are overweight or obese. "It's a pretty easy prevention effort to just turn off the lights before you go to bed", Ms Sandler said. A normal or healthy BMI is typically considered to be between 18.5 and 24.9.

The women's self-reported sleeping habits were put into four categories: no light, small nightlight in the room, light outside of the room, and light or television in the room.

Artificial light exposure at night from things like streetlights, storefronts, and even cell phone use, for example, can disrupt the natural light-dark cycle of circadian rhythms and suppress the release of the sleep hormone melatonin.

However, researchers were quick to point out that exposure to artificial light at night can be indicative of socioeconomic disadvantage or unhealthy behaviors, which could contribute to weight gain and obesity. The women also answered questions about their level of exposure to light at night while sleeping, such as light from other rooms, light from outside, light from a TV, or light in the bedroom.

Professor Malcolm von Schantz, from the University of Surrey, commented on the study, saying: 'The findings make flawless biological sense. "We know from experimental studies in people that light at night affects our metabolism in ways that are consistent with increased risk of metabolic syndrome", he said.

'These new findings won't change the advice to maintain good sleep hygiene, and avoid light and electronic distractions in the bedroom, but they add further strength to the case for this advice'.